"So many people enter and leave your life! Hundreds of thousands of people! You have to keep the door open so they can come in! But it also means you have to let them go!"

Mr. Black says this quotation to Oskar while talking about his life in Chapter 7. It serves as a warning to Oskar about the double bind of love—that it’s both necessary and painful. Mr. Black specifically writes about war, making him an expert on the loss that comes with it. Therefore, this deceptively simple advice offers Oskar a perspective on his sorrow that ties his grief to that of other historical wars. This quotation also contradicts Oskar’s mom’s request that he stop giving his key out to strangers because he could get hurt. Mr. Black, despite knowing how terrible people can be, believes that the potential of meeting wonderful people makes the risk worthwhile. Accordingly, throughout their friendship, Mr. Black encourages Oskar to drop the rules that he believed kept him safe, like not taking public transit, in favor of seeing new parts of the city and experiencing people he never would have otherwise. However, Mr. Black also leaves Oskar, forcing him to figure out a way to move forward without him.

This truism also cuts to the heart of Thomas’s great struggle to move forward and positions Mr. Black as a foil to Thomas. Thomas responds to Anna’s loss by locking himself deep inside, shutting up his true emotions and feelings, as symbolized by the doorknob images in his notebooks. In his relationship with Grandma, whom he never refers to by name, he tries to use her as a replacement for Anna. When he asks Grandma to pose for him, he transforms her into Anna in the resulting sculpture, literally trying to recreate his lost love through Grandma. Thomas closes his door—and his heart—to the possibility of future love because he cannot let go of the past. In contrast, although Mr. Black shuts himself up for years, he allows Oskar to bring him out into the world and even finds a possibility of new love with Ruth Black. When Mr. Black learns of Thomas’s existence, he exits Oskar’s life so that Oskar can move on to forging a relationship with his real grandpa, not a surrogate.